Diffusers



Sept. 29, 1964 T. E. ADAMS 3,150,823

DIFFusERs Filed Feb. 7, 1963 musuraa THON A5' EDwRD ADAMS United States Patent Office 3,159,823 Patented Sept. 29, 1964 DIFFUSERS Thomas Edward Adams, Rugby, England, assigner to Associated Electrical industries Limited, London, England, a company of Great Britain Filed Feb. 7, 1963, Ser. No. 256,985 Claims priority, application Great Britain Feb. 12, 1%2 S Claims. (Cl. 230-132) The present invention relates to diffusers which are used in centrifugal compressors to decrease the velocity and increase the pressure of a fluid after the fluid has passed through the impeller of the compressor.

The fluid will be exhausted from exit apertures in the radially outer periphery of the impeller and the diffuser is conveniently of annular form and fits around the outer periphery of the impeller. If the axial length of the impeller outlet is small then the axial length at the en trance to the diffuser is also small.

The diffuser comprises a plurality of channels gradually Aincreasing in cross-sectional area in the direction of flow of the fluid generally arranged symmetrically about the axis of the impeller. Each channel comprises a collector region near to the impeller outlet, and a diffuser region located downstream from the collector region. All or most of the diffusion is performed in the diffuser region, while the collector region serves to collect fluid from the impeller outlet apertures and lead it to the diffuser region. The fluid may diffuse to a small extent in the collector region.

The efficiency of a properly designed diffuser depends on the cross-sectional shape of each channel, the etliciency being a function of the ratio of the area to the peripheral length of any cross-section in a plane at right angles to the direction of flow. Therefore the most ef* ficient diffuser has channels of substantially circular cross-section. However, such a did'user when used with Y an impeller in a compressor is complicated in construction and expensive to manufacture.

A diffuser which is less efficient but simpler and cheaper to manufacture has channels of substantially square crosssection. A diffuser with channels of substantially rectangular cross-section has an efllciency which is less than that of a diffuser with square cross-section channels but in certain circumstances it is even simpler to manufacture.

lt is possible to make a diffuser with each channel having a substantially square or rectangular cross-section throughout its length by using a plurality of vanes extending from the exit apertures of the impeller, but if the axial length of the iin-peller outlet is small and the impeller outlet diameter is big then a large number of vanes will be reouired to define channels even approaching a square cross-sectional shape. The greater the ratio between diameter of the impeller at outlet and the axial length of Athe outlet passage, the greater is the number of diffuser vanes required for the maximum fluid pressure rise in the diffuser. The results in the formation of a large number of channels which although in general give good fluid pressure increase through the diffuser, have other serious aerodynamic disadvantages.

The object of the present invention is to provide an improved diffuser which has a form of construction which is simple and cheap.

According to the present invention a diffuser for use with a centrifugal compressor comprises a plurality of channels, each of substantially rectangular cross-section and having a collector region and a diffuser region, as defined, the ratio of the lengths of the circumferential side and the axial side, as defined, of the input end of the collector region being greater than the ratio of the lengths of the two corresponding sides of the input end of the diffuser region, and the mean rate of increase in the direction of mean fluid flow of the length of the axial side of each channel being greater in the collector region than in the diffuser region.

According to a preferred embodiment of the invention the cross-section of the diffuser region of each channel is substantially square.

The terms circumferential and axial are used with respect to the axis of the impeller with which the diffuser is adapted to be used.

ln a diffuser in accordance with the invention the collector region of each channel provides a transition between an inlet to each channel in which the ratio of the lengths of the circumferential sides to the axial sides is large, and a diffuser region of substantially square or rectangular cross-section in which the ratio of the lengths of the corresponding sides is less.

Since collection of fluid takes place in the collector region, the mean rate of increase of the length of the axial side of the channel in the collector region can be greater than that permitted for a diffuser region where only diffusion and no collection of fluid takes place. The rate of increase, in .the direction of fluid flow, of the lengths of the axial sides of the channels may be constant, or may be varied so that the axial sides of the channels may be curved in the collector regions or in the diffuser regions or both.

The diffuser may be formed from two spaced apart coaxial annular plates, the adjacent surfaces of the two plates being divided into radially inner portions and radially outer portions, the radially outer portions being inclined to each other at a rst angle of inclination and the radially inner portions being incline-:l to each other at a second angle of inclination which is greater than said first angle of inclination, together with a plurality of varies extending between said adjacent surfaces so as to define said channels extending from the inner peripherie-s of said annular plates, the inner and outer portions of the adjacent surfaces of the plates generally defining respectively the collector and diduser regions of the channels.

in an alternative construction the radially inner portions of the adjacent surfaces of the plates are curved relative to one another.

In order that the invention may be more readily understood reference will now be made to the accompanying drawing in which:

FIG. l is a front view partly in section of a diffuser assembly embodying the invention;

FlG. 2 is a side View of part of the assembly' illustrated in FIG. l sectioned on the plane Il ll; and

FIG. 3 is a side view of an alternative form of the Iasserlrbly illustrated in FIG. 1 sectioned on the plane With reference to FIGS. l and 2 the diffuser assembly is formed from two annular plates 1, 2, which are placed coaxially with their common axis extending perpendicularly into the plane of the figure, but are spaced apart. The adjacent surfaces of the two plates are divided into radially outer portions 3, 4 and radially inner portions 5, 6. The radially outer portions are inclined to each other at a first angle of inclination. The radially inner portions 5, 6 are inclined to each other at a second angle of inclination which is greater than said first angle of inclination. In the alternative construction illustrated in FIG. 3 the radially inner portions 5, 6 are curved so that their radially outer parts are inclined to each other at a smaller angle than the radially inner parts.

The two plates l, 2 may be identical or one plate may be formed with a flat surface facing the other plate and the angles of inclination between the adjacent surfaces may be arranged by suitably shaping the surface of the other plate. l

A plurality of vanes 7, 8 extend between the adjacent surfaces of the annular plates. These vanes are curved in radial directions relative to the axis of the plates, as seen in FIG. 1 but are flat in axial directions as seen in FIG. 2. The vanes define a plurality of diffuser channels 9, extending from the inner peripheries of the plates, 1, 2 forming diffuser regions in combination with the radially outer portions 3, 4 of the adjacent surfaces of the plates 1, 2 and collector regions in combination with the radially inner portions 5, 6 of the adjacent surfaces of the plates 1, 2.

The vanes 7, 8 are so shaped that the cross-section of each diffuser channel is as required. As pointed out above the nearer the cross-section of each channel approaches a square the more efficient will be the diffuser. The diffuser is designed to be used with a centrifugal impeller which is located in the cylindrical space 12 within the plates 1, 2 and extends coaxially within the plates. The collector region of each channel provides a transition between its inlet and the corresponding diffuser region. At the inlet to the collector region the ratio of the lengths of the circumferential side to the axial side may be very large as indicated above and greater than the required ratio of the lengths of the corresponding sides of the diffuser region.

In order to provide the transition, the lengths of the axial sides of each collector region are designed to increase rapidly in the direction of rneans flow of the fiuid so that they rapidly reach the value of the required lengths of the corresponding axial sides of the diffuser region. This rapid increase is provided by the angle of inclination of the inner portions 5, 6 of the adjacent surfaces of the plates 1, 2. The vanes '7, 8 extend in gradual curves to define the Widths of the other sides of the channels.

With this arrangement, a transition is provided between each inlet aperture and a corresponding diffuser region by a simple construction of the collector region. Since collection of fluid with little or no diffusion takes place in the collector region, the axial sides thereof can diverge at an angle which is greater than that permitted for efficient diffusion in the diffusing region where there is no further collection of fluid into the channel.

The sides of the collector region may be curved as illustrated in FIG. 3 and the sides of the diffuser region may also be curved. In order to provide this arrangement a small number of vanes of simple shape are used and steady flow of the fluid throughout the diffuser is maintained.

It will be appreciated that with the diffuser described above a simple transition is provided between the outlet of the impeller and the diffuser regions of the channels. This simple transition is still provided even if each inlet has short axial sides and long circumferential sides while the cross-section of the diffuser region more approaches a square.

What I claim is:

1. A diffuser for use with a centrifugal compressor comprising a casing, surfaces to said casing defining a plurality of channels each of substantially rectangular cross section, and a collector region and a diffuser region, as defined, to each channel, the ratio of the lengths of the circumferential side and the axial side, as dened, of the input end of the collector region being greater than the ratio of the lengths of the two corresponding sides of the input end of the diffuser region, and the mean rate of increase in the direction of mean fluid flow of the length of the axial side of each channel being greater in the collector region than in the diffuser region.

2. A diffuser for use with a centrifugal compressor comprising a casing, surfaces to said casing defining a plurality of channels each of substantially rectangular cross section, and a collector region and a diffuser region, as defined, to each channel, each diffuser region having a substantially square cross section, the ratio of the lengths of the circumferential side and the axial side, as defined, of the input end of the collector region being greater than the ratio of the lengths of the two corresponding sides of the input end of the diffuser region, and the mean rate of increase in the direction of mean fiuid flow of the length of the axial side of each channel being greater in the collector region than in the diffuser region.

3. A diffuser for use with a centrifugal compressor comprising two spaced apart coaxial annular plates, the adjacent surfaces of said plates being divided into radially inner portions and radially outer portions, the radially outer portions being inclined to each other at a rst angle of inclination and the radially inner portions being inclined to each other at a second angle of inclination which is greater than said first angle of inclination, together with a plurality of vanes extending between said adjacent surfaces so as to define a plurality of channels extending from the inner peripheries of said annular plates, the radially inner and outer portions of the adjacent surfaces of the plates generally defining respectively the collector and the diffuser regions of the channels.

4. A diffuser for use with a centrifugal compressor comprising two spaced apart coaxial annular plates, the adjacent surfaces of said plates being divided into radially inner portions and radially outer portions, the radially outer portions being inclined to each other at a first angle of inclination and the radially inner portions being curved so that their radially outer ends are inclined to each other at said first angle of inclination and their radially inner ends are inclined to each other at a second angle of inclination which is greater than said first angle of inclination, together with a plurality of vanes extending between said adjacent surfaces so as to dene a plurality of channels extending from the inner peripheries of said annular plates, the radially inner and outer portions of the adjacent surfaces of the plates generally defining respectively the collector and the diffuser regions of the channels.

5. In a centrifugal compressor wherein a centrifugal impeller is centrally disposed in a space within a pair of spaced coaxial annular plates having opposed inner surface portions diverging from said space and merging into opposed outer diverging surface portions, the outer portions each diverging at a smaller included angle than the inner portions, and a plurality of vanes extending from said space between the adjacent plates curved in the radial direction and defining a plurality of fluid flow channels extending outwardly from said space, said vanes forming with said inner plate surface portions collector regions each of which provides a transition region between the flow channel inlet that receives fluid from the impeller to a diffuser region formed by said vanes and said outer plate surface portions.

References Cited in the file of this patent UNITED STATES PATENTS 1,745,854 Lawaczeck Feb. 4, 1930 2,609,141 Aue Sept. 2, 1952 2,658,338 Leduc Nov. 10, 1953 2,900,126 Dangerfield Aug. 18, 1959 FOREIGN PATENTS 152,689 Great Britain Oct. 21, 1920 170,815 Great Britain Oct. 14, 1921 253,302 Great Britain June 17, 1926 

1. A DIFFUSER FOR USE WITH A CENTRIFUGAL COMPRESSOR COMPRISING A CASING, SURFACES TO SAID CASING DEFINING A PLURALITY OF CHANNELS EACH OF SUBSTANTIALLY RECTANGULAR CROSS SECTION, AND A COLLECTOR REGION AND A DIFFUSER REGION, AS DEFINED, TO EACH CHANNEL, THE RATIO OF THE LENGTHS OF THE CIRCUMFERENTIAL SIDE AND THE AXIAL SIDE, AS DEFINE, OF THE INPUT END OF THE COLLECTOR REGION BEING GREATER THAN THE RATIO OF THE LENGTHS OF THE TWO CORRESPONDING SIDES OF THE INPUT END OF THE DIFFUSER REGION, AND THE MEAN RATE OF INCREASE IN THE DIRECTION OF MEAN FLUID FLOW OF THE LENGTH OF THE AXIAL SIDE OF EACH CHANNEL BEING GREATER IN THE COLLECTOR REGION THAN IN THE DIFFUSER REGION. 